1. Field of the Invention
This invention relates to the minimizing of metal-mold reactions between cast superalloy and directionally solidified eutectic materials and cores/or molds employed therewith.
2. Description of the Prior Art
The production of directionally solidified (DS) metal eutectic alloys and superalloys for high pressure turbine (HPT) airfoils with intricate internal passageways for air cooling requires that the core and mold not only be dimension-alloy stable and sufficiently strong to contain and shape the casting but also be sufficiently crushable to prevent mechanical rupture (hot cracking) of the casting during solidification and cooling. The DS process requirements of up to 1875.degree. C. for a 16 hr. time period imposes severe constraints on materials which may serve as mold or core candidates.
The prior art appears to be mostly limited to the use of silica or silica-zircon core and mold materials. At temperatures greater than 1600.degree. C. the silica based materials fail from the standpoint of both mechanical integrity and chemical incompatibility with the advanced alloy compositions.
Dimensional control of the silica core is excellent since cristobalite exhibits very little densification. Microstructural examination reveals that, in some cases, commercial core compositions employ very large particles (&gt;100 .mu.m). The addition of large particles serves to lower both shrinkage and mechanical strength.
Paul S. Svec in "Process For Making An Investment Mold For Casting And Solidification Of Superalloys Therein", Ser. No. 590,970, teaches the use of alumina-silica compositions for making molds and cores. Charles D. Grekovich and Michael F. X. Gigliotti, Jr. in U.S. Pat. Nos. 3,955,616, 3,972,367, and 4,031,945 teach cores and molds of alumina-silica, yttria-silica, and magnesium-silica compositions which have a barrier layer formed at the mold/metal interface. In particular they discuss alumina barrier layers. One possible means for the formation of their alumina layer is by a chemical reaction wherein carbon of the susceptor chemically reduces the material composition of the mold or core. It is also taught that when other alloys not having aluminum therein may be cast in their molds provided they contain such materials as magnesium, yttrium, hafnium, zirconium and titanium, therein.
When a rare-earth doped NiTaC-13 alloy which contain aluminum in its compositions, was cast in an alumina-silica mold, a severe metal-mold reaction occurred which caused severe surface defects in the surface of the casting.
Charles D. Greskovich, Ser. No. 698,909, also teaches an alumina-silica composition wherein the material is of a predetermined size so as to favor, and therefore enable, the formation of metastable mullite for molds and cores which exhibit superior sag resistance at high temperatures.
Aluminum oxide (Al.sub.2 O.sub.3) by itself, without a chemical or physical binder material, has been identified as a potential core and mold material based on both chemical compatibility and leachability considerations. Aluminum oxide, or alumina doped with rare earth oxides has also been proposed as suitable core and mold materials for casting and solidifying the same superalloy materials. Some oxide ceramics have a free energy of formation that is much more negative than that of Al.sub.2 O.sub.3 and yet react with the aluminum of a superalloy material such as NiTaC-13 and the like. Such a reaction results in the formation of a mixed oxide layer at the metal-ceramic interface. In particular when Y.sub.2 O.sub.3 is the ceramic material in contact with the molten metal NiTaC-13, the double oxides Y.sub.3 Al.sub.5 O.sub.12, YAlO.sub.3 and Y.sub.4 Al.sub.2 O.sub.9 are formed at the metal-ceramic interface when Y.sub.2 O.sub.3 reacts with molten metal. Such a reaction layer is undesirable for several reasons including:
1. Such a mixed oxide layer which forms during directional solidification has an irregular surface morphology. Removal of the mixed oxide layer results in a rough metal finish for the casting and loss of dimensional tolerances.
2. The reaction layer is usually very difficult to leach from the casting by the autoclave caustic leaching process.
3. The reaction layer does not appear to dissolve in mineral acids.
Therefore, it is an object of this invention to provide a new and improved method for minimizing the formation of a reaction layer at the metal-ceramic interface when a superalloy is cast and directionally solidified.
Another object of this invention is to provide a new and improved method for minimizing the formation of a reaction layer at the metal-ceramic interface by doping a superalloy material with a rare earth metal whose metal cation is in the ceramic oxide material comprising the mold and/or core.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.